Roughly half of the U.S. wheat crop is exported, and most of it is used to make food, including breads, pastries, and noodles.

USDA officials said the Food and Drug Administration determined that there is no health risk to humans from the modified strain.

"Hopefully, our trading partners will be very understanding," Michael Scuse, the acting deputy agriculture secretary, said at a briefing. Major customers for U.S. wheat had been informed of the discovery over the past day, he said.

Genetically modified crops cannot be grown legally in the U.S. unless the federal government approves them after a review to ensure that they pose no threat to the environment or to people.

Monsanto entered four strains of glyphosate-resistant wheat for approval in the 1990s, but regulators never provided a final ruling because the company decided it had no market.

The genetically modified wheat sprouted this spring in a field that grew winter wheat last year. When the farmer sprayed the so-called volunteer plants with a glyphosate herbicide, some of them unexpectedly survived. Samples were then sent to Oregon State University and to the USDA for analysis.

Testing showed the wheat was a Monsanto-developed strain resistant to glyphosate. The company is assisting in the investigation, the department said, adding that Monsanto tested Roundup Ready wheat varieties—those resistant to the widely used herbicide—in 16 states from 1998 to 2005.

Scuse and Michael Firko, who oversees the USDA's biotechnology approval process, said the department was looking into how the strain appeared on the farm when no seeds should have been available for several years.

"I think it will have a significant impact," said Ronnie Cummins, national director of the Organic Consumers Association, which battled to keep genetically modified wheat out of the marketplace.

The Senate last week rejected by a wide margin a measure to let states order labeling of food made with genetically engineered crops. Cummins said the discovery of the rogue plants in Oregon would accelerate efforts to require GE food labels.